1. Field of the Invention
The present invention relates to a digital image processing system and a digital mammography system.
2. Description of the Related Art
As is well known, it has been performed to execute gradation processing of a digital radiation image, especially a digital X-ray image, according to a body part of a subject and/or a diagnostic purpose with a digital radiation image processing system.
The image which has received gradation processing is printed on a film to be output as a hard copy, or is output to an image display apparatus such as a CRT. Then, the output image is supplied to a reading (interpretation of image) by a doctor.
In case of a fluorescent intensifying screen-film system, a relation between an X-ray exposure amount and a developed density is always constant. However, in a digital radiation image processing system, an image reading apparatus converts an image signal to a digital signal within a range necessary as the image signal. Consequently, the digital radiation image processing system can control which density the digital signal is reproduced in. The control is called as gradation processing.
In the following, as an example, conventional normalization of a histogram and conventional gradation processing are described with reference to FIGS. 4 and 5.
First, representative values D1 and D2 are set from a cumulative histogram of image data in a region of interest (ROI). The representative values D1 and D2 are set as the levels of image data at which the cumulative histogram takes predetermined rates.
When the representative values D1 and D2, or the levels of image data at which the cumulative histogram takes predetermined rates m1 and m2, respectively, are set, normalization processing of performing the level conversion of the representative values D1 and D2 to desired levels S1 and S2, respectively, as shown in FIG. 4, is carried out by referring to a previously set. normalization processing look-up table. In FIG. 4, the ordinate axis indicates levels, and the abscissa axis indicates radiation doses. Hereupon, a characteristic curve CC shows the levels of the signals output according to the radiation doses of the radiations radiated to a radiation image conversion panel-through a subject.
Moreover, the normalization processing look-up table is generated by operations using the inverse function of a function indicating the characteristic curve CC of the radiation image conversion panel. Incidentally, the normalization processing may be performed by operation processing without using the normalization look-up table.
Next, gradation processing is performed using normalized image data DTreg obtained by the normalization processing. In the gradation processing, for example, a gradation conversion curve shown in FIG. 5 is used, and the normalized image data DTreg is converted into output image data DTout based on the levels S1′ and S2′ converted from reference values S1 and S2 of the normalized image data DTreg, respectively. The levels S1′ and S2′ severally correspond to predetermined luminance or density in an output image.
Generally, when a conversion of a digital signal based on a gradation conversion curve is performed, the conversion is adapted to previously store an output signal value corresponding to each input signal value as a series of data row (gradation conversion table), and to obtain an output signal value by referring to the data row whenever an input signal value is given.
Incidentally, because the shape of a preferable gradation conversion curve and the levels S1′ and S2′ differ according to a radiographing body part, a radiographing positioning, radiographing conditions, a radiographing method and the like, the gradation conversion curve may be produced to every image at each time.
Moreover, it is supposed that a plurality of basic gradation conversion curves are stored beforehand, and a desired gradation conversion curve can be easily obtained by reading any of the basic gradation conversion curves to perform a rotation and/or a parallel movement thereof.
As image processing, the output image data DTout which has received a desired gradation conversion can be obtained also by providing a gradation conversion table corresponding to a plurality of basic gradation curves, and by referring to the gradation conversion table based on the normalized image data DTreg while performing conversion to correct the obtained image data according to a rotation and/or a parallel movement of the basic gradation conversion curve.
The selection of a basic gradation curve, the rotation and/or the parallel movement of the basic gradation curve is performed based on the kind of the image display apparatus or the information pertaining to the kind of an external apparatus for outputting an image as the need arises. The reason is that a preferable gradation may differ dependently on the output system of the image.
As an example of the effects of the gradation processing, a contrast is improved in case of changing a gradation conversion curve to the one by which a difference of input image signals is changed to a larger density difference or a larger luminance difference in an output image.
As stated in JP-3260153B, it is performed to emphasize a region necessary to be improved in contrast according to a diagnosis purpose in a digital radiation image processing system. For example, in a chest X-ray image, the contrast of the region corresponding to a lung field is made to be the highest, and the contrast of the region of a mediastinum is suppressed not to skip to white. By such a way, reproduction is frequently performed so as to obtain a proper density over a wide range of an image signal. This is the gradation processing which can obtain almost the same gradation characteristic as that of the conventional fluorescent intensifying screen-film system or a little higher contrast of the region of a lung field.
Although the image having such a gradation characteristic is a mean one and is suitable for a standard diagnosis, there are demands of a diagnosis in a state of a still higher contrast in the lung field, a diagnosis in a state of a higher contrast in the mediastinum portion, and the like. In particular, in case of a group examination, a subject does not have symptoms of which the subject is conscious, and the existence or the kind of a change to a morbid state cannot be expected at all. The invention stated in JP-3260153B is constructed so as to switch LUT data in a gradation conversion unit while repeatedly transferring the LUT data to a display unit through the gradation conversion unit in the state of being synchronized with the display unit and the gradation conversion unit to make it possible to instantaneously obtain images which have received different kinds of gradation processing only by switching the LUT data in the gradation unit in order to perform a diagnosis using a plurality of images reproduced by the different kinds of gradation processing effectively.
However, even the prior art mentioned above still has the following problems.
A gradation conversion curve “a” corresponding to the gradation conversion table used in the conventional digital radiation image processing system is shown in FIG. 2. A gradient curve “a′” of the conventional conversion curve “a” is shown in FIG. 3.
As is well confirmed by the gradient curve “a′”, the gradient of the conventional gradation conversion curve “a” changes from an increase to a decrease through the maximum point. The reason why the conventional gradation conversion curve “a” has such a characteristic is that the gradation conversion curve “a” follows the gradation characteristic of the conventional fluorescent intensifying screen-film system under the apprehension of a sense of incompatibility with a established diagnosing system using fluorescent intensifying screen-film system.
Because the contrast is improved as the gradient of the gradation conversion curve becomes larger, the conventional digital radiation image processing system sets the maximum point of the gradient of the gradation conversion curve “a” (=the maximum point of the gradient curve “a′”) within a density range of a body part which is watched most strongly from the viewpoint of a diagnosis.
Consequently, the conventional gradation conversion curve “a” has a problem in which it becomes more difficult to perform a conversion in a good contrast over the whole of the body part in proportion as the density range of the body part becomes wider.
Moreover, the object body parts important from the viewpoint of a diagnosis are not always one in a medical image. In a medical image, the density ranges of a plurality of object body parts may differ from each other to be wide as a whole. Consequently, the conventional gradation conversion curve “a” has a problem in which it is difficult to perform conversions in all of the object body parts in good contrasts when a plurality of the objects body parts exist.
For example, in mammography, the gradation characteristic has been determined to make the contrast in the most important mammary gland from the viewpoint of a diagnosis best, although low density microcalcification, and a fat region and a pectoral region, which are high density regions, are also very important regions. Consequently, the contrasts of the microcalcification, the fat layer and the pectoral region may be lowered.
When the contrast of the whole is increased in vain in order to raise the contrast before and behind the highest point of the gradient of the conventional gradation conversion curve “a”, the contrast may exceed the reproduction ability of a display device such as a CRT, or it may become necessary to especially prepare a Schaukasten (light box) of high luminance in a film reading system.
When a radiographed image is converted using different gradation conversion tables in order to output a plurality of images the gradations of which have been converted to be the optimum for each object body part, interpretation comparing each body part becomes troublesome. Although the technique stated in JP-3260153B can be used in case of outputting to an image monitor, the interpretation comparing each body part becomes especially troublesome in case of outputting to a film.
Even if a plurality of images which has received different kinds of gradation conversion processing is output, or even if an image each image region of which has received different kinds of gradation conversion processing is output, attention is needed when the comparing radiogram interpretation is performed to portions to which different gradation conversion characteristics have been applied, and a new radiogram interpretation standard taking the differences of the gradation conversion characteristics into account is required of a reading doctor. Consequently, it is apprehended that it becomes difficult for a radiogram interpretation doctor having deep experience in the fluorescent intensifying screen-film system radiograph to perform a diagnosis.
By the way, a mammography image has a narrow dynamic range, and in a low gradation expression, it is impossible to obtain a mammography image the diagnosis of which can be performed, and a radiogram interpretation of a comparatively minute change becomes necessary. Also in the mammography system, digitization has been progressing with a new technique such as a phase contrast radiographing technique in recent years. However, a display output to an image monitor, which has a less gradation number as compared with a film output, of the mammography image, which has a narrow dynamic range, is seldom used, and the film output is solely performed.
Accordingly, in a digital mammography system, it is desired that the film outputting of an image in which each diagnosis object body part is converted into a good contrast contributes to the state of a diagnosis being proper and expedient.